1. Field of the Invention
The present invention relates generally to gyroscopes and more particularly to vibratory gyroscopes.
2. Description of the Related Art
Gyroscopes are devices which can sense angular rotation and/or rotation rate. Accordingly, they are useful in situations (e.g., satellite attitude control) where observation of other inertial indicators (e.g., cosmic bodies such as the sun) is temporarily obscured.
A variety of gyroscope concepts have been developed. For example, gyroscopes based upon gimballed spinning wheels and gyroscopes based upon laser rings have been shown to be highly accurate. Although these devices find use in numerous applications (e.g., inertial navigation), their high expense and large size discourage a wider use.
In contrast to these gyroscopes, the concept of vibratory gyroscopes is based on rotation-induced energy exchange between modes of vibrating members. This concept is exemplified by an analysis of ringing wine glasses that was performed in 1890 by G. H. Bryan. In a flexural mode, the lip of a wine glass vibrates in elliptical-shaped modes that have two nodal diameters. When the wine glass is rotated, Bryan found that the node lines lag behind (precess) the rotation of the wine glass (e.g., during a 90.degree. rotation, the node lines were observed to precess by .about.27.degree.). This nodal lag is, therefore, an indication of angular rotation.
Although highly accurate hemispherical resonator gyroscopes have been built using the wine glass example (e.g., see Wright, David, et al., "The HRG Applied to a Satellite Attitude Reference System", from Guidance and Control by Culp, R. D., et al., American Astronautical Society, 1994, volume 86, pp. 57--63), their nonplanar form is difficult to miniaturize and requires complicated, expensive fabrication processes.
Other nonplanar vibratory gyroscope structures have been investigated (e.g., see Putty, Michael W., et al., "A Micromachined Vibrating Ring Gyroscope", Solid-State Sensor and Actuators Workshop, Jun. 13-16, 1994, pp. 213-220). For example, cantilevered beams have been used to form vibratory gyroscopes. Experience with these devices has shown them to be difficult to mount and to be sensitive to temperature and spurious vibrations. To overcome the difficulties of cantilevered beams, tuning fork gyroscopes have been developed. These are balanced devices which are easier to mount and less sensitive to linear vibrations. However, fabrication and temperature drift problems limit the matching of input and output mode frequencies which, in turn, degrades the gyroscope's sensitivity. Misalignment of mass centers can also produce an undesirable vibration response which causes bias errors.
In contrast to these vibratory gyroscope types, the cost and size of planar vibratory gyroscopes is relatively low because they are mechanically simple (e.g., there is an absence of rotating parts) and their design typically facilitates miniaturization and batch fabrication with micromachining techniques. In addition, the precision of micromachining has enabled many vibratory gyroscopes to achieve impressive accuracy.
One conventional planar vibratory gyroscope employs a vibrating ring as its sensing element (e.g., see Johnson, Jack D., et al., "Surface Micromachined Angular Rate Sensor", 1995 SAE Conference Paper 950538, pp 77-83). This ring element can be considered to be a slice out of Bryan's wine glass. In a controlled resonance, the ring assumes an elliptical pattern in which four nodes on the ring have no deflection and four antinodes on the ring are each located between a pair of nodes and exhibit maximal radial deflection. In response to rotation, the angular position of the nodes lags the angular position to which the gyroscope is rotated.
Another planar vibratory gyroscope is typically referred to as a clover-leaf gyroscope (e.g., see Tang, Tony K., et al., "Silicon Bulk Micromachined Vibratory Gyroscope", 1996 Solid-State Sensor and Actuator Workshop, Hilton Head, S.C., June 2-6) because it has a planar member whose outline resembles a four leaf clover. This member is suspended by four thin wires or beams from a housing and a metal post is coupled to the center of the member with an orientation orthogonal to the member's plane. The thin clover leaves provide large areas for electrostatic driving and capacitive sensing.
The resonator is electrostatically excited in a control mode to rotate about a first axis of the planar member which causes the post to move in a second axis of the planar member that is orthogonal to the first axis. In response to a rotation about a third axis that is orthogonal to the member's plane, the motion of the oscillating post is displaced into movement along the first axis. This post displacement translates into a sense mode rotation of the planar member about the second axis. Essentially, the post couples energy between the control and sense modes.
Although the planar vibratory gyroscopes described above can be miniaturized and can be generally realized with low-cost micromachining techniques, they suffer from various operational defects. For example, the ring gyroscope is planar and symmetric but the sensitivity of its control and sense electrodes is degraded because of the small electrode size required to couple to the ring's flexing perimeter. In addition, the ring gyroscope's circular form degrades the precision with which it can be defined in bulk crystalline material by photographic masks. As a second example, the orthogonally mounted post of the clover-leaf gyroscope detracts from its otherwise planar configuration. The post requires a manual assembly procedure which typically degrades the gyroscope's symmetry. In addition, this gyroscope's narrow beam supports are a source of high stress and nonlinearity.